1. Field of the Invention
The present invention relates generally to the field of oxygen scavenging polymers. More particularly, it concerns emulsions of oxygen scavenging polymers, especially branched or partially crosslinked oxygen scavenging polymers.
2. Description of Related Art
It is well known that limiting the exposure of oxygen-sensitive products to oxygen maintains and enhances the quality and shelf-life of the product. For instance, by limiting the oxygen exposure of oxygen sensitive food products in a packaging system, the quality of the food product is maintained, and food spoilage is avoided. In addition such packaging also keeps the product in inventory longer, thereby reducing costs incurred from waste and restocking.
A growing technique for limiting oxygen exposure involves incorporating an oxygen scavenger into the packaging structure. The oxygen scavenger is typically a polymer capable of reacting irreversibly with oxygen that also provides adequate structural properties alone or as a blend in an oxygen scavenging layer of a packaging article. Many early approaches involved the use of polymers comprising linear alkenyl moieties, either in the polymer backbone or in groups pendant on the backbone, as oxygen scavengers. However, these approaches often suffered from off-taste and malodor concerns raised by the migration of fragments of the linear alkenyl moieties (generated by the scavenging reaction) into a packaged food or beverage.
A milestone in the development of oxygen scavenging polymers was reached by Ching et al., International Patent Publ. No. WO99/48963, which discloses the use of polymers comprising an ethylenic backbone and cycloalkenyl pendant groups, especially polymers comprising cycloalkenyl methyl acrylate or cycloalkenyl methyl methacrylate units, as oxygen scavenging polymers. The cycloalkenyl pendant groups do not generate fragments upon undergoing the scavenging reaction, and thus impart minimal off-taste and malodor to a packaged food or beverage.
The polymers comprising cycloalkenyl methyl (meth)acrylate disclosed by Ching et al. are typically formed by transesterification of acrylates or methacrylates by alcohols comprising the cycloalkenyl methyl moiety in a reactive extrusion process. Such techniques are effective; however, the process of producing the oxygen scavenging polymer via transesterification through reactive extrusion is expensive.
Production of polymers comprising cycloalkenyl methyl (meth)acrylate units by the free-radical-based direct polymerization of cycloalkenyl methyl (meth)acrylate monomers has not been shown. We found that free-radical-based bulk polymerization or solution polymerization of such monomers would lead to excessive crosslinking involving the double bonds of the cycloalkenyl groups. The crosslinked polymer would not be suitable to process further in the formation of an oxygen scavenging packaging article from the polymer. Also, excessive consumption of the double bonds of the cycloalkenyl groups would lower the oxygen scavenging capacity of the polymer to undesirably low levels.
Therefore, it would be desirable to have an oxygen scavenging polymer, especially one comprising cycloalkenyl pendant groups, that can be used to form an oxygen scavenging packaging article without the requirement for a solid formulation of same. Further, it would be desirable to have a free-radical based polymerization method that does not lead to excessive crosslinking of double bonds present in pendant groups, especially cycloalkenyl pendant groups. It would be further desirable that organic solvents are not required in the performance of the method.
In one embodiment, the present invention relates to a branched or crosslinked polymer. Preferably, the branched or crosslinked polymer comprises structure I: 
wherein R1 and R2 are each selected from xe2x80x94H, xe2x80x94CH3, or a monomeric unit of a polymer wherein the monomeric unit comprises an ethylenic backbone and a cyclic hydrocarbon pendant group, provided at least one of R1 and R2 is the monomeric unit; M is xe2x80x94(CR32)nxe2x80x94, wherein n is an integer from 0 to 4, inclusive; X is null or a linking group; and each R3 is independently selected from xe2x80x94H or xe2x80x94CH3.
In another embodiment, the present invention relates to an emulsion, comprising an oxygen scavenging polymer, water, and a surfactant. The oxygen scavenging polymer may be branched or crosslinked.
In still another embodiment, the present invention relates to an oxygen scavenging packaging article, comprising a structural layer and a polymer coating comprising an oxygen scavenging polymer and a surfactant. The oxygen scavenging polymer may be branched or crosslinked.
In yet a further embodiment, the present invention relates to a method for preparing an emulsion comprising an oxygen scavenging polymer, water, and a surfactant, the method comprising providing water, the surfactant, monomeric components of the oxygen scavenging polymer, and a free radical initiator; forming a first emulsion comprising water, the free radical initiator, and the surfactant; adding the monomeric components to the first emulsion; and polymerizing the monomeric components at a temperature at least about 70xc2x0 C. for about 2-6 hr, to form the emulsion comprising an oxygen scavenging polymer, water, and a surfactant. The oxygen scavenging polymer may be branched or crosslinked.
In still a further embodiment, the present invention relates to a method of forming an oxygen scavenging packaging article, comprising providing a packaging article; applying an emulsion comprising an oxygen scavenging polymer, water, and a surfactant at least one surface of the packaging article, to form an emulsion-coated surface; and evaporating water from the emulsion-coated surface, to form the oxygen scavenging packaging article. The oxygen scavenging polymer may be branched or crosslinked.
In yet an additional embodiment, the present invention relates to a method of forming an oxygen scavenging packaging article comprising two adhered surfaces, comprising providing a packaging article comprising two surfaces which are desired to be adhered; applying an emulsion comprising an oxygen scavenging polymer, water, and a surfactant to at least one of the two surfaces of the packaging article, to form an emulsion-coated surface; contacting the two surfaces which are desired to be adhered; and evaporating water from the emulsion-coated surface, to adhere the two surfaces and form the oxygen scavenging packaging article. The oxygen scavenging polymer may be branched or crosslinked.
The present invention provides a formulation of an oxygen scavenging polymer other than a solid formulation. The present invention also provides a method of forming an oxygen scavenging polymer via free-radical-based polymerization. Further, organic solvent is not required in the production of the oxygen scavenging polymer.